1. Field of the Invention
The present invention relates to a thermistor element which can detect a temperature ranging from room temperature to high temperature of about 1000xc2x0 C., i.e. so-called wide-range type thermistor element, and the thermistor element is particularly suitable for use in a temperature sensor for an automobile exhaust gas.
2. Description of the Related Art
A thermistor element for a temperature sensor is used in the measurement of a temperature ranging from moderate to high temperature (e.g. 400 to 1300xc2x0 C., etc.) such as temperature of an automobile exhaust gas, gas flame temperature of gas hot-water supply device, temperature of a heating oven, etc.
Characteristics of this kind of a thermistor element are indicated by the resistivity and resistivity temperature coefficient (temperature dependence of the resistivity). In order to cope with a practical resistivity range of a temperature detecting circuit constituting the temperature sensor, it is desired that the resistivity of the thermistor element is within a predetermined range. Therefore, perovskite materials are exclusively used as those having resistivity characteristics suitable for a wide-range type thermistor element.
As the thermistor element using perovskite material, for example, those described in Japanese Patent Kokai Publication Nos. Hei 6-325907 and Hei 7-201528 are suggested. These thermistor elements are produced by mixing oxides of Y, Sr, Cr, Fe, Ti, etc. in a predetermined composition proportion and calcining the mixture to form a perfect solid solution in order to realize a thermistor which can used in a wide temperature range.
The resistivity characteristics of the wide-range type thermistor element are indicated by the resistivity and resistivity temperature coefficient. In a normal temperature sensor, it is necessary that the resistivity of the thermistor element is from 50 to 300 kxcexa9 within a working temperature range in view of the resistivity range of the temperature detecting circuit. In case of affording a heat history from room temperature to 1000xc2x0 C. to the thermistor element, the smaller a change between the resistivity after heat history and the initial resistivity, the better.
In the above Japanese Patent Publications, various thermistor elements of a perfect solid solution are suggested, but only data of the thermistor element resistivity at 300xc2x0 C. or more are disclosed. Therefore, the present inventors have examined the resistivity characteristics at about room temperature of various thermistor elements in the above Japanese Patent Publications.
As a result, regarding those having a resistivity stability in the heat history from room temperature to 1000xc2x0 C., the resistivity becomes higher in the temperature range from room temperature to 300xc2x0 C. Therefore, it is impossible to discriminate it from insulation and the temperature can not be detected. On the other hand, regarding those satisfying low resistivity of 50 to 300 kxcexa9, the resistivity changes by 10% or more relative to the initial resistivity in the heat history. It has been found that the stability is poor.
There has never been obtained a thermistor element which can satisfy two resistivity characteristics which are contrary to each other, i.e. low resistivity characteristics within a range from room temperature to high temperature of 1000xc2x0 C. and resistivity stability in the heat history (so-called wide-range type thermistor element).
In the light of the above problems, an object of the present invention is to provide a thermistor element which has stable characteristics (i.e. small change in resistivity in the heat history from room temperature to 1000xc2x0 C.) and has a resistivity of 50 to 300 kxcexa9 within the temperature range from room temperature to 1000xc2x0 C.
(A) In the first aspect of the present invention for accomplishing the above object, the present inventors have considered that a conventional thermistor element is composed of a perfect solid solution having a perovskite type structure but it is difficult for a perfect solid solution as a single compound to satisfy the above resistivity characteristics which are liable to be contrary to each other.
Thus, the above object has been accomplished by using a novel thermistor material composed of a mixed sintered body prepared by mixing two compounds, i.e. a perovskite material (oxide) having a comparatively low resistivity and a material having a comparatively high resistivity in place of the perfect solid solution.
The present inventors have tested and studied various perovskite materials. As a result, it has been found that a composition M1M2O3 (M1 is at least one element selected from the elements of the groups II and IIIA excluding La in the Periodic Table, and M2 is at least one element selected from the elements of the groups IIB, IIIB, IVA, VA, VIA, VIIA and VIII) is preferable as a material having resistivity characteristics which are suitable for accomplishing the above object.
Since La has high moisture absorption property, there is a problem that La reacts with water in the air to form an unstable hydroxide, which results in breakage of the thermistor element. Therefore, La is not used as M2.
On the other hand, it has been decided that Y2O3 (yttrium oxide), which has a comparatively high resistivity and stabilizes resistivity of the thermistor material, is used as another material to be mixed, as a result of the study.
By preparing a mixed sintered body from M1M2O3 and Y2O3, a thermistor element of a mixed sintered body M1M2O3.Y2O3. The term xe2x80x9cmixed sintered bodyxe2x80x9d used herein means a sintered body wherein grains constituting the sintered body comprise a mixture of grains of a first component M1M2O3 and grains of a second component Y2O3.
1) That is, this mixed sintered body is a mixed sintered body M1M2O3.Y2O3 of the above M1M2O3 and Y2O3, wherein M1 is at least one element selected from the elements of the groups IIA and IIIA excluding La in the Periodic Table, and M2 is at least one element selected from the elements of the groups IIB, IIIB, IVA, VA, VIA, VIIA and VIII in the composition M1M2O3. More specifically, it can also be represented as aM1M2O3.bY2O3.
This thermistor element was incorporated into a temperature sensor and the resistivity characteristics of the element were examined. As a result, it could be confirmed that it is stable, that is, a change in resistivity is small (e.g. few %, etc.) even in the heat history from room temperature to 1000xc2x0 C. and the resistivity is from 50 to 300 kxcexa9 within the temperature range from room temperature to 1000xc2x0 C.
Therefore, according to this invention, it is possible to provide a thermistor element which can detect a temperature ranging from room temperature to high temperature of 1000xc2x0 C. and has stable characteristics, that is, a change in resistivity is small even in the heat history from room temperature to 1000xc2x0 C., so-called wide-range type thermistor element.
2) As a result of the study of the present inventors, regarding each element in the above perovskite compound M1M2O3, M1 is preferably at least one element selected from Y, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb, Mg, Ca, Sr, Ba and Sc, and M2 is preferably at least one element selected from Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Al, Ga, Zr, Nb, Mo, Hf, Ta and W, in view of the practical use.
3) As a result of a further study about a mixing ratio of M1M2O3 and Y2O3, it has been found that the effect of the present invention can be accomplished more certainly if the mixing ratio is within a predetermined range, that is, when a molar fraction of the above M1M2O3 is a and b is a molar fraction of the above Y2O3, these molar fraction a and b satisfy the relations 0.05xe2x89xa6a less than 1.0, 0 less than bxe2x89xa60.95 and a+b=1 in the composition formula aM1M2O3.bY2O3.
Since the molar fractions can be changed within a wide range in such way, the resistivity and resistivity temperature coefficient can be widely controlled by appropriately mixing both M1M2O3 and Y2O3 and firing the mixture.
4) In the sintered body, a sintering auxiliary is added to improve the sintering property of the respective particles. As a result of the test and study about various sintering auxiliaries, it has been found that it is preferable to use a sintering auxiliary comprising at least one of CaO, CaCO3 and CaSiO3, and SiO2 in case of the mixed sintered body of the present invention. Consequently, a wide-range type thermistor element having excellent sintering density can be obtained.
(B) As a result of the advancement of the test, it has been found that a detected temperature accuracy varies with the sensor in the level within the range from xc2x120 to 30xc2x0 C. in the temperature sensor using the above thermistor element.
Hence, an examination of various conditions in the production step of the thermistor element, such as compounding, molding and firing or sintering conditions have been advanced for the purpose of improving this temperature accuracy (reduction of scatter in detected temperature accuracy between sensors).
As a result, it has been found that scatter in temperature accuracy arises as follows. That is, since the average particle diameter of M1M2O3 as the perovskite material obtained by the calcination is larger than that of Y2O3, both components are not uniformly mixed to cause scatter in composition of the mixed sintered body, which results in scatter in resistivity of the thermistor element.
1) Therefore, the present inventors have considered that uniform mixing of the composition can be realized if the average particle diameter of M1M2O3 is adjusted to an average particle diameter which is the same as that of Y2O3 in the mixed state before calcination, and a test and study have been made. As a result, it has been found that M1M2O3 obtained by the calcination and Y2O3 may be mixed and ground to adjust the average particle diameter of this mixture (M1M2O3 and Y2O3) to an average particle diameter which is not more than that of Y2O3 before mixing.
That is, when using this production method, uniform mixing is realized by atomization of M1M2O3 and Y2O3 and a variation in composition of the mixed sintered body M1M2O3.Y2O3 is reduced and, therefore, scatter in resistivity of the thermistor element can be reduced. Accordingly, it is possible to provide a wide-range type thermistor element which can realize a sensor temperature accuracy better than a conventional level within the temperature range from room temperature to 1000xc2x0 C. (small scatter in temperature accuracy between sensors).
2) The mixed sintered body Y(CrMn)O3.Y2O3 can also be obtained by a method of mixing an oxide of Cr with an oxide of Mn, calcining the mixture at 1000xc2x0 C. or more to obtain (Mn1.5Cr1.5)O4, and performing direct mixing/sintering of (Mn1.5Cr1.5)O4 and Y2O3 in place of a method of mixing Y(CrMn)O3 with Y2O3 and sintering the mixture. In this case, the same effect can be exerted by mixing an oxide of Cr with an oxide of Mn, calcining the mixture at 1000xc2x0 C. or more to obtain (Mn1.5Cr1.5)O4 having an average particle diameter larger than that of the above Y2O3, mixing this (Mn1.5Cr1.5)O4 with the above Y2O3, grinding the mixture to adjust the average particle diameter of this mixture to an average particle diameter which is not more than that of the above Y2O3 before mixing, molding the mixture into an article having a predetermined shape and sintering the article.
3) The mixed sintered body Y(CrMnTi)O3.Y2O3 can also be obtained by mixing an oxide of Cr with an oxide of Mn, calcining the mixture at 1000xc2x0 C. or more to obtain (Mn1.5Cr1.5)O4, and performing mixing and sintering of (Mn1.5Cr1.5)O4, Y2O3 and TiO2. In this case, the same effect can be obtained by mixing an oxide of Cr with an oxide of Mn, calcining the mixture at 1000xc2x0 C. more to obtain (Mn1.5Cr1.5)O4 having an average particle diameter larger than that of the above Y2O3, mixing this (Mn1.5Cr1.5)O4 with the above Y2O3 and TiO2, grinding the mixture to adjust the average particle diameter of this ground mixture to an average particle diameter which is not more than that of the above Y2O3 before mixing, molding the mixture into an article having a predetermined shape and sintering the article.
(C) Furthermore, an examination of the production method of the thermistor element has been advanced for the purpose of improving the detected temperature accuracy of the temperature sensor using the thermistor element of the present invention. As a result, it has been found that scatter in composition of M1M2O3 itself obtained by the calcination exerts an influence on scatter in composition of the mixed sintered body M1M2O3.Y2O3 (i.e. scatter in resistivity of the thermistor element).
Now, the cause of scatter in the composition of M1M2O3 obtained by the calcination in the method of producing the mixed sintered body M1M2O3.Y2O3 will be described by way of the example wherein M1=Y and M2=Cr and Mn, i.e. example using Y(Cr0.5Mn0.5)O3.
For example, Y(Cr0.5Mn0.5)O3 is prepared as follows (see FIG. 20). Y2O3 (average particle diameter: about 1 xcexcm) as a source material of M1, and Cr2O3 (average particle diameter: about 4 xcexcm) and Mn2O3 (average particle diameter: about 7 xcexcm) as source materials of M2 are compounded in a molar ratio Y:Cr:Mn=1:0.5:0.5 (compounding 1), mixed and ground by using a ball mill, and then this mixture is calcined at 1000xc2x0 C. or more to obtain Y(Cr0.5Mn0.5)O3.
The present inventors have found that a problem lies in the mixing and grinding using a ball mill in the above step. That is, according to the mixing and grinding using a ball mill, the average particle diameter after the mixing and grinding is limited to about 2 xcexcm and the average particle diameter of Cr2O3 and that of Mn2O3 are larger than that of Y2O3.
Accordingly, Y(Cr0.5Mn0.5)O3 obtained by the calcination reaction of the mixture of Y2O3, Cr2O3 and Mn2O3 becomes a mixture containing a composition shifted from Y:Cr:Mn=1:0.5:0.5 due to a difference in particle diameter of each raw material, e.g. various compositions from composition of Y:Cr:Mn=1:0.6:0.4 to composition of Y:Cr:Mn=1:0.4:0.6.
Since these compositions, from a composition of Y:Cr:Mn=1:0.6:0.4 to a composition of Y:Cr:Mn=1:0.4:0.6, have different resistivity and resistivity temperature coefficient (xcex2 value), the resistivity varies with the element to cause scatter in element resistivity.
In case that a part of Y2O3, Cr2O3 and Mn2O3 as the raw material (shifted from the composition ratio) is remained as an unreacted matter, scatter in element resistivity arises.
The present inventors have intensively studied problems such as scatter in composition of M1M2O3 obtained in the step before obtaining M1M2O3 by the calcination, presence of the unreacted matter, etc.
As a result, it has been found that the above drawbacks can be inhibited and the temperature accuracy becomes xc2x110xc2x0 C. or less if the raw material of M2 and that of M2 are mixed and ground by using a medium stirring mill having a grinding capability higher than that of a ball mill and atomization is performed so that the average particle diameter of the raw material mixture (mixed grind) after mixing and grinding is adjusted to an average particle diameter which is not more than that of the raw material of M1 and is not more than 0.5 xcexcm.
The method of producing the thermistor element of the present invention has been accomplished based on the above finding.
1) That is, in this invention, the raw material of M2 and the raw material of M1 are mixed and ground to adjust the average particle diameter of this mixed grind to an average particle diameter which is not more than that of the raw material of M1 before mixing and is not more than 0.5 xcexcm in the mixing step of mixing and grinding the raw material of M2 and the raw material of M1. Thereafter, M1M2O3 is obtained by calcination, and the M1M2O3 and Y2O3 are then mixed. The mixture is molded into an article having a predetermined shape and then sintered.
According to the present invention, since uniform mixing of the composition can be realized by uniform atomization of the raw materials of M1 and M2, reduction of scatter in composition of M1M2O3 formed after calcination and inhibition of the existence of the raw material unreacted reaction matter can be realized.
Therefore, scatter in resistivity of the thermistor element can be reduced.
Accordingly, it is possible to provide a wide-range type thermistor element which realizes a sensor temperature accuracy better than a conventional level within the temperature range from room temperature to 1000xc2x0 C. (small scatter in temperature accuracy between sensors).
When using those containing at least Y2O3 as the raw material of M1, a thermistor element can also be obtained by mixing the raw material of M1 and the raw material of M2, grinding the mixture, calcining the mixture to form a precursor having the same composition as that of the desired mixed sintered body M1M2O3.Y2O3, molding this precursor into an article having a predetermined shape, and sintering the article.
The precursor is represented by M1M2O3.Y2O3, wherein Y2O3 containing excess Y in an amount larger than a theoretical amount is combined with M1M2O3 in the above M1M2O3 (perovskite structure). Therefore, according to this production method, a mixed sintered body, i.e. thermistor element can be obtained by previously compounding the raw materials of M1 and M2 so that the composition of the desired mixed sintered body can be obtained without further adding Y2O3 after calcination.
2) In addition, according to the production method using precursors containing at least Y2O3, the above precursor is obtained by mixing the raw material of M1 and the raw material of M2, grinding the mixture to adjust the average particle diameter of this mixed grind to an average particle diameter which is not more than that of the raw material of M1 before mixing and which is 0.5 xcexcm or less, and calcining the mixed grind.
Consequently, since uniform mixing of the composition can be realized by uniform atomization of the raw materials of M1 and M2, reduction of scatter in composition of the precursor formed after calcination and inhibition of the existence of the raw material unreacted reaction matter can be realized. As a result, scatter in composition of the mixed sintered body having the same composition as that of the precursor can be reduced and the same effect as that of the above item 1) can be obtained.
3) A method of mixing a raw material of M2 with a raw material of M1, grinding the mixture to adjust an average particle diameter of the mixed grind to an average particle diameter which is not more than that of the raw material of M1 before mixing and is also not more than 0.5 xcexcm, calcining the ground mixture to obtain M1M2O3,
mixing M1M2O3 obtained by the calcination with Y2O3, grinding the mixture to adjust an average particle diameter of the mixture after grinding to an average particle diameter which is not more than that of the raw material of Y2O3 before mixing, molding the ground mixture into an article having a predetermined shape, and sintering the article is a combination of the above production method (B) and (C), and this method can reduce scatter in resistivity of the thermistor element to a higher level by a combination of the effects of both methods.
4) Similarly, a method of using precursors containing at least Y2O3 as a raw material of M1, mixing a raw material of M2 with the raw material of M1, grinding the mixture to adjust an average particle diameter of the mixed grind after grinding to an average particle diameter which is not more than that of the raw material of M1 before mixing and is also not more than 0.5 xcexcm, calcining the ground mixture to obtain a precursor having the same composition as that of the mixed sintered body M1M2O3.Y2O3,
grinding the precursor obtained by the calcination to adjust an average particle diameter of the precursor after grinding to an average particle diameter which is not more than that of the raw material Y2O3 as the raw material of M1 before mixing, molding the ground precursor into an article having a predetermined shape, and calcining the article is a combination of the above production method (B) and (C). According to this method, since the uniform mixing of M1M2O3 and Y2O3 can be realized and a variation in composition of the mixed sintered body can be reduced in the molding and sintering as the following step, by atomizing the precursor M1M2O3.Y2O3 having the same composition as that of the mixed sintered body to a level smaller than the average particle diameter of Y2O3 as the raw material of M1, scatter in resistivity of the thermistor element can be reduced.
That is, according to this element, scatter in resistivity of the thermistor element can be reduced to a higher level.
The material of the above conventional thermistor element is a perfect solid solution having a perovskite type structure. In case of a YCrO3 perovskite type material, the valence of a Y ion of A site or Cr of a B site ion is controlled by other element ions so as to optionally control the resistivity and resistivity temperature coefficient. The present inventors have considered that the crystal structure becomes unstable by increasing the substitution element ions according to this method and it is difficult to satisfy the resistance characteristics which are liable to be contrary to each other.
Hence, the present inventors have decided to accomplish the above object by selecting the element capable of controlling the resistivity and resistivity temperature coefficient of a wide-range type thermistor element which can maintain the stability of the crystal structure and realize the stability of the resistivity even in the heat history, in a small amount to be substituted.
1) The present inventors have tested and studied various perovskite materials. As a result, it has been found that a novel composition M1(M2M3)O3 (M1 is at least one element selected from the elements of the groups II and IIIA excluding La in the Periodic Table, and IV and M3 respectively represent at least one element selected from the elements of the groups IIB, IIIB, IVA, VA, VIA, VIIA and VIII, wherein the relation of 1 less than b less than 0.1 is satisfied when a molar fraction of M2 is a, a molar fraction of M3 is b and a+b=1 in M1(M2M3)O3) is preferable as a material having resistivity characteristics which are suitable for accomplishing the above object.
Since La has high moisture absorption property, there is a problem that La reacts with water in the air to form an unstable hydroxide, which results in breakage of the thermistor element. Therefore, La is not used as M1.
This wide-range type thermistor element was incorporated into a temperature sensor and the resistivity characteristics of the element were examined. As a result, it could be confirmed that it is stable, that is, a change in resistivity is small even in the heat history from room temperature to 1000xc2x0 C. and the resistivity is from 60 to 300 kxcexa9 within the temperature range from room temperature to 1000xc2x0 C.
Therefore, according to the above invention, it is possible to provide a wide-range type thermistor element which can detect a temperature ranging from room temperature to high temperature of 1000xc2x0 C. and has stable characteristics, that is, a change in resistivity is small even in the heat history from room temperature to 1000xc2x0 C.
2) As a result of the study of the present inventors, regarding each element in the above perovskite compound M1(M2M3)O3, M1 is preferably at least one element selected from Y, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb, Mg, Ca, Sr, Ba and Sc, and M2 and M3 preferably represent at least one element selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Ga, Zr, Nb, Mo, Zr, Hf, Ta and W, in view of practical use.
3) Furthermore, it has been found that the above effect can be accomplished, more certainly, if the relation of 1 less than b less than 0.1 is satisfied when a molar fraction of M2 is a, a molar fraction of M3 is b and a+b=1 in perovskite compound M1(M2M3)O3 where M1 is Y, M2 comprises Cr and M3 and M3 is Ti, i.e., Cr(MnTi)O3.
4) In the sintering of the above compound M1(M2M3)O3, a sintering auxiliary is added to improve the sintering property of the respective particles. As a result of the test and study of various sintering auxiliaries, it has been found that it is preferable to use a sintering auxiliary comprising at least one of CaO, CaCO3 and CaSiO3, and SiO2 in case of the sintered body of the invention of this aspect. Consequently, according to this aspect, a thermistor element having excellent sintering density in the above compound M1(M2M3)O3 can be obtained.